Well, Darkmoor, of course, but I'm not sure that's at the top of my list. I find myself avoiding dungeons whose layout is in some way confusing, or that take a long time at the appropriate level. I hardly ever do the abandoned temple beyond Andover Portobello's quest in New Sorpigal. I also avoid the Hall of the Fire Lord because I can figure out how to get around in there and I always have to go back and forth multiple times in the basement areas of Varn, because I just can't remember the layout. As to outdoor areas there are a few that are tough, but I think I mostly dread clearing Frozen Highlands because of the harpies on the mountains, which makes getting cursed doubly hard to avoid. I wonder what everyone else's take is on this.
By the way, I'm playing through again, partly because I always enjoy it, but also because with a steadily fading memory it's a good test. It's been about 4 months since I last played a game and both remembering which quests to do and the layout of dungeons - plus trying to remember where I was and what I was doing yesterday when I load my game again - is a good benchmark. The latter is my bigger problem, so I guess it's mostly short term memory that's fading, which is not as big a concern.
